Porosity and the fatigue behavior of hypoeutectic and hypereutectic aluminum-silicon casting alloys

The fatigue behavior of five Al-Si casting alloys under the influence of microstructural heterogeneities was investigated for the present study. The silicon content of these alloys ranged from 7 to 17%, and includes hypoeutectic LP PM319-F, A356-T6 and C354-T6 alloys and hypereutectic AE425 and PM390 alloys, as used extensively in automotive applications. Compression-tension fatigue tests were carried out applying a stress ratio of R = -1, and a sinusoidal waveform, under high cycle (> 10(4)) fatigue conditions at room temperature (25 degrees C) for the hypoeutectic alloys and at a high temperature (300 degrees C) for the hypereutectic alloys. The fracture surfaces of the specimens tested were examined using a scanning electron microscope (SEM) to determine the fatigue crack initiation sites. Quantitative measurements of the defect characteristics associated with the fatigue crack initiation site were carried out using an image analyzer system. A correlation was made between the sample fatigue life and the size of the microstructural defect which initiated the fatigue crack. Porosity was observed to have the greatest propensity for acting as a crack initiation site in these casting alloys. It is, thus, mainly responsible for decreasing the fatigue life of these castings, since 90% of all samples examined tended to fracture as a result of this microstructural defect. As regards the casting alloys examined, it was observed that the fatigue strength decreases as the size of the pore increases, and vice versa. The influence of microstructural characteristics on the fatigue performance of these alloys and the fatigue behavior of defect-free castings. as opposed to that of castings with defects, were presented elsewhere. (c) 2007 Elsevier Ltd. All rights reserved.